Many or most of the short-span overpass bridges in the United States are constructed of a deck surface on top of a supporting structure, most commonly a framework of steel or concrete I-beams. For example, a conventional four-span, two-lane overpass bridge built in the late 1940's across a four-lane divided highway (a total span of approximately 215 feet) could have a 3" pavement wearing surface on a 7" structural slab of reinforced concrete supported on top of a framing system consisting of five longitudinal 33" deep steel I-beams.
One relatively new example of a bridge design using a deck surface on top of a supporting structure can be found in Barnoff et al., U.S. Pat. No. 4,604,841, in which a deck surface takes the form of precast, prestressed concrete form panels that are supported by floor beams. In one illustration in the patent, the floor beams have the form of conventional I-beams. Another example of a bridge design using a deck surface on top of a supporting structure can be found in Schupack, U.S. Pat. No. 3,794,433, in which the deck surface rests on top of concrete, open-topped box beams. Similarly, Hewett, U.S. Pat. No. 890,769 discloses a bridge in which a deck surface is supported on top of integral concrete ribs.
All of these designs have a relatively thick profile between the top of the deck surface and the bottom of the supporting structure. This thickness limits the clearance below the bridge, which, in overpass bridges, can be extremely important. In many instances in the U.S., when existing bridges need to be replaced because of deterioration or no longer meeting applicable highway standards, it may be necessary, based on applicable highway standards, to provide a new bridge with greater clearance. It is not uncommon for engineers to obtain greater clearance in a replacement bridge by simply raising the grade of the roadway approaching the bridge. Such a solution is expensive and inconvenient, particularly if other structures are already present in the area.
There is believed to be a significant need in the U.S. for a bridge that offers higher clearance over existing roadways without the need to raise the grade of the roadway approaching the bridge, and that can be built not only at a competitive cost, but also with a minimal disruption of traffic on the roadway below.
It has been known that prefabrication of bridge elements is advantageous. Thus, for example, Slaw, Sr., U.S. Pat. No. 4,972,537, discloses a composite prefabricated deck panel that may be used in bridges or buildings. The composite panel consists of a concrete slab poured over the top flanges of a series of I-beams. As another example, Richard, U.S. Pat. No. 4,625,354, discloses a two-deck bridge made of prefabricated sections that are stressed by cables that pass through the hollow interiors between the structural walls of each section. While the advantages of using prefabricated segments have been known, it is believed that it has not been known how to apply these principals to the problem of increasing the clearance under a short-span overpass bridge.